1. Field of the Invention
The present invention relates to a single lens reflex camera in which the light from an object, passing through a photographing lens, is guided through a half mirror to a photoelectric converting element, and the signal therefrom is used for detecting object information utilized in photographing, such as focus information and light measurement information.
2. Related Background Art
As disclosed in the U.S. Pat. No. 4,047,022 and in the Japanese Patent Publication No. 60-32846, there is already known a focus detecting device using the so-called collephoto method, in which the focus state of an imaging lens is detected by placing a slit member having plural slit apertures of a predetermined pitch in the vicinity of the anticipated focal plane of the imaging lens or a plane equivalent thereto, vibrating said slit member in a direction perpendicular to the optical axis of said imaging lens or a direction of the array of said slit apertures, and determining the phase difference of two images of the object, entering a set of photosensors positioned therebehind.
However the distance measurement in such a device is limited to the center of the viewing field, namely to an area on the optical axis of the imaging lens. Therefore, the distance measurement can be conducted without problem if the object of focus detection is positioned at the center of the viewing field, but appropriate focusing operation cannot be achieved as precise distance information is not obtained if the object is not at the center of the viewing field at the distance measuring operation. In consideration of this fact, assignee of the present applicants already proposed, in U.S. patent application Ser. No. 162,575, a device enabling focus detection at a desired position by moving the distance measuring area in the viewing field.
However, if such proposed device is to be incorporated in the body of a single lens reflex camera, with an optical system arrangement consisting of a photographing lens, a main movable mirror, an auxiliary movable mirror and a focus detecting system positioned in the bottom of the mirror box as employed in the conventional single lens reflex camera, the movable range of the measuring area in the viewing field is limited by the size or position of the auxiliary movable mirror. Thus, in practice, there is encountered a case where said distance measuring area can be moved only in a very limited range.
FIG. 14 is a cross-sectional view of a conventional single lens reflex camera in which the light coming from the object passes through a photographing lens 101, is deflected by a movable mirror 102 to form an image of the object on a focusing plate 103, is then diffused therein and enters a condenser lens 104 positioned therebehind. The light is divided into two portions, one being reflected by a half mirror, then wholly reflected by the bottom of the condenser lens 104 and guided to a photosensor sensor 105, while the other is transmitted by said half mirror and guided through a pentagonal prism 106 to an eyepiece 107.
In the camera of such structure, the vertical dimension of the camera can be reduced without imparing the functions by placing said condenser lens 104 in front of the focusing plate 103, if such dimensional reduction is required for example by the design. However, in such structure with the condenser lens 104 positioned between the photographing lens 101 and the focusing plate 103, the image of the object focused on said focusing plate 103 is distorted, so that the focusing state can be confirmed at the central portion of the viewing field.
Also the assignee of the present applicants has disclosed, in another patent application, a camera provided with a light reflecting surface in a part of the light entering face (imaging face) of the focusing plate or an equivalent surface, wherein the light reflected from said reflecting surface is guided to a focus detecting system (or a light measuring system) provided at the bottom of the mirror box, for detecting the information on distance (or on light intensity). However, such camera is also associated with a drawback of distorted image of providing a the object on the focusing plate, because the condenser lens is positioned between the photographing lens and the focusing plate, for example in order to utilize said condenser lens both in the finder system and in the focus detecting system.